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Substantial amounts of the transition metals Mn, Fe, Co, and Ni can be substituted for Li in single crystalline Li$_2$(Li$_{1-x}T_x$)N. Isothermal and temperature-dependent magnetization measurements reveal local magnetic moments with magnitudes significantly exceeding the spin-only value. The additional contributions stem from unquenched orbital moments that lead to rare-earth-like behavior of the magnetic properties. Accordingly, extremely large magnetic anisotropies have been found. Most notably, the magnetic anisotropy alternates as easy-plane $\rightarrow$ easy-axis $\rightarrow$ easy-plane $\rightarrow$ easy-axis when progressing from $T$ = Mn $\rightarrow$ Fe $\rightarrow$ Co $\rightarrow$ Ni. This behavior can be understood based on a perturbation approach in an analytical, single-ion model. The calculated magnetic anisotropies show a surprisingly good agreement with the experiment and capture the basic features observed for the different transition metals., Comment: 5 pages, 3 figures, published as PRB Rapid Communication, Fig. 3 updated

Large magnetic anisotropy and coercivity are key properties of functional magnetic materials and are generally associated with rare earth elements. Here we show an extreme, uniaxial magnetic anisotropy and the emergence of magnetic hysteresis in Li2(Li1-xFex)N. An extrapolated, magnetic anisotropy field of 220 Tesla and a coercivity field of over 11 Tesla at 2 Kelvin outperform all known hard-ferromagnets and single-molecule magnets (SMMs). Steps in the hysteresis loops and relaxation phenomena in striking similarity to SMMs are particularly pronounced for x << 1 and indicate the presence of nano-scale magnetic centres. Quantum tunnelling, in form of temperature-independent relaxation and coercivity, deviation from Arrhenius behaviour and blocking of the relaxation, dominates the magnetic properties up to 10 Kelvin. The simple crystal structure, the availability of large single crystals, and the ability to vary the Fe concentration make Li2(Li1-xFex)N (i) an ideal model system to study macroscopic quantum effects at elevated temperatures and (ii) a basis for novel functional magnetic materials., Comment: 19 pages, 5 figures, Supplementary Information attached, revised version published in Nature Communications